I am currently using the PIC18LF4550 coupled with MATRIX Multiprogramming Board (EB-006).
After building the firmware hex file in MPLAB, i using PPP programmer(this works for the MATRIX Board) to burn the hex to PIC micro. I also configure the configuration bits as specified in Main.c
The system detects the PIC micro as a HID device (about 4 times in 5 attempts). I have also built the C# program and when i run the application file it detects the connected device for a second and then hangs for 20 seconds and finally returns "device detached". I tried replacing the 19.6608 Mhz that comes with the multiprogramming Board with a 20.064 Mhz crystal but the result remains the same.

PS: The LED connected to PIC micro remains lit the whole time.

It's very difficult to know what's going wrong in cases such as this without having the same board to test. The error you are seeing (with the hanging) is because the device enumerates and then does not respond to the host when a command is passed to it which is why the host application detaches the device.

It could be anything from a floating reset line on the PIC or a faulty USB cable.

The easiest suggestion (since the original project circuit is very simple) is to build the original circuit on a breadboard and then test your hex file to see if it works. That way you know if it's the firmware or your target hardware which is causing the issue.

Is it safe to assume PIC18F4550 and PIC18LF4550 are same as far as this project is considered?

Also i am using a different programmer(MATRIX). Do you think it will make any difference ?

If I remember right the LF is the low-power version, but as long as you are running at 5Vs it shouldn't make a difference. The programmer shouldn't affect anything either.